Fact-based soybean aphid insecticide recommendations

Since soybean aphids’ introduction, a tremendous amount of research and observational data has been obtained. We now have the tools and the knowledge to effectively manage the pest.

Information sources

Soybean producers have a wide array of pest management advice and information available, so the question is: Where to get the best information?

The internet is particularly rife with newsletters, social media postings and videos all purporting to give expert advice. It’s wise to always consider the source of the information and also evaluate what it’s actually based on—making a statement with absolute certainty doesn’t necessarily make it a fact.

Research and review process

The land-grant university system and the Extension mission were created to conduct unbiased research and provide education for the public good.

As scientists at universities, we make pest management recommendations based on repeated and controlled studies, statistical tests and, ultimately, a peer review system that ensures what we publish is thoroughly vetted and evaluated by other scientists, often anonymously.

However, for many information sources available to soybean farmers, there’s no review of any kind. As a result, many of the “recommendations” from entities not relying on sound science are never challenged or critically evaluated. They’re just opinions.

Recommendations backed by science

These recommendations aren’t just anecdotal, or based on hunches and feelings. They’re based on:

• Facts.

• Established crop and pest biology.

• Effectiveness of single or combined management tactics.

• Short- and long-term economic costs.

• Environmental implications.

None of what we present on this page is new or groundbreaking information. However, all of what we’re presenting is based on science that’s been vetted and implemented over thousands of acres for more than a decade.

Farmers often find the “free” application costs when insecticides are tank-mixed with herbicides or other pesticides have unintended negative consequences—poor control from poor timing or application techniques of one or more products.

Using an ET based on sound, peer-reviewed research will help you apply your crop input dollars where they’re most likely to produce a positive return on your investment and minimize the chances of creating other problems.

What to watch for

Using fear or faulty economic logic is tried and true as a very effective sales tool. It’s always prudent to be a skeptical consumer and consider the messenger when you evaluate information—a conflict of interest can arise if a profit motive underpins recommendations made without facts behind them.

Be very wary of ETs based on “feel,” eyewitness accounts or other anecdotes that aren’t supported by hard scientific data. ETs that are radically different from those recommended by agricultural research universities are another red flag.

The soybean aphid feeds on the phloem fluids (sometimes referred to as sap) by inserting piercing-sucking mouthparts directly into the phloem vessels that carry products of photosynthesis from the leaves to other plant parts. Prior to feeding, aphids “taste” the sap to determine if the plant is a suitable host species and if the quality is acceptable.

Once they settle and begin feeding, injury from soybean aphid infestations can reduce:

• Plant growth.

• Pod number.

• Seed number.

• Seed weight.

• Seed oil concentration.

• Photosynthesis rates.

Early and prolonged aphid infestations can affect all yield components, while later infestations only tend to reduce seed size.

Feeding duration and pressure

Direct yield loss from soybean aphid feeding doesn’t occur when the first (or five or ten) aphids begin feeding. Today’s soybean varieties are equipped to handle minor challenges, including a few aphids.

Yield loss from soybean aphid is related to how many soybean aphids are present and for how long the aphids are present and feeding. Aphid population pressure over time is calculated as aphid-days.

Simply put, this is the average number of aphids on a plant multiplied by the number of days they’re present. A single soybean aphid on a plant for 10 days is equal to 10 aphid-days, 200 aphids on a plant for 20 days is equal to 4,000 aphid-days, and so on. This aphid-day concept has proven to be a good indicator of how soybean yield responds to aphid populations.

Sap and honeydew

Feeding by aphids doesn’t cause the plant to “leak sap.” Soybean aphids require specific nitrogen-rich amino acids that are present in plant fluids at low concentrations. This means aphids must consume large volumes of sap to acquire enough nutrition.

Aphids excrete the excess water and sugars from the sap as waste that’s called honeydew. It’s the sticky, shiny substance that accumulates on leaves of aphid-infested plants.

The sugary honeydew is sometimes fed on and used by other insects as an energy source. A fungus called sooty mold also uses aphid honeydew and results in a dark coating on soybean leaves that may block sunlight and interfere with photosynthesis.

Diseases and viruses

Soybean aphids aren’t known to transmit fungal or bacterial diseases to soybean. Because soybean aphids and soybean pathogens are associated with certain environments, some wrongly assume that the presence of a fungal disease (such as charcoal rot) means the aphids transmitted the disease or that the disease entered through the wounds caused by aphids.

However, aphid feeding can transmit disease-causing viruses from one plant to another. The soybean aphid has been associated with the transmission of several viral soybean diseases, such as soybean mosaic virus and alfalfa mosaic virus.

Because these viral diseases currently aren’t considered significant threats to soybean yield in the north-central U.S., they’re not directly accounted for in general aphid management recommendations.

The “tasting” or probing of plants by soybean aphids can also transmit viruses in plants that aren’t soybean aphid hosts, such as dry beans and potato. This effect in other crops is particularly pronounced when soybean aphid populations are high.

When aphids cause yield loss

The lowest level of aphid infestation shown to cause yield loss in soybean is several thousand aphid-days. This value, referred to as the damage boundary, is a biological relationship between the insect, crop and environment, and is independent of crop and input costs.

Below the damage boundary, there’s no measurable damage. This is why efforts to treat aphid levels well below the damage boundary can’t provide a return on investment.

When yield loss equals management costs

The economic injury level (EIL) is the point at which yield loss from insect damage equals the cost of a management action, such as an insecticide application. Applying insecticide to pest populations that haven’t reached this point, and are unlikely to reach it, wouldn’t provide any return.

To more readily apply this yield-loss relationship to field scouting and aphid management, scientists calculated an aphids per plant value as the threshold to apply an insecticide to threatening populations (Figure 1).

When to take action

The economic threshold (ET), also referred to as an action threshold or treatment threshold, is the key number for managing the pest. When the insect population reaches this point, you should take action to prevent a growing population from causing economic injury.

Use the following economic threshold; all three conditions should be true before treating:

• Average of 250 aphids per plant.

• More than 80 percent of plants having aphids.

• Aphid populations increasing.

In most insect pest thresholds, including soybean aphid, the ET occurs well before the EIL to minimize the grower’s chance of incurring economic loss. In fact, the ET for soybean aphid occurs before the damage boundary.

In addition to costs, the ET may take into account factors like insecticide effectiveness, rate of insect reproduction, crop development and lead times for insecticide application.

How values were determined

All above values and statements are based on data, and determined by closely monitoring aphid populations in research plots.

Thousands of whole-plant aphid counts were taken at frequent intervals throughout the growing season to determine the soybean aphid values for damage boundary, EIL and ET. The large multi-state dataset that went into these calculations included a wide range of soybean-growing environments, including moisture variation and other stresses.

New research on pest and crop biology and on new management tools may change EILs and associated ETs over time. However, it’s been about a decade since scientists developed and published the current threshold, and university research continues to confirm these values.

Continued validity

However, some have questioned the continued validity of the original soybean aphid ET of 250 aphids per plant, which was calculated with economic conditions from the mid-2000s. Calculations using different economic conditions—such as for the current year—but without regard to biology (which hasn’t changed) may suggest using a lower ET for aphid management. This is based on faulty logic.

Aphid biology and how the plant reacts to aphids make the exercise meaningless. No economic gain can be found by treating at those lower aphid numbers (remember the ET of 250 aphids per plant is already well below the damage boundary), and low numbers of aphids often don’t reach the EIL.

While economic injury levels account for commodity prices, labor and control costs, the levels’ biological components aren’t sensitive to commodity or input prices. Your farm’s insects don’t eat faster or more when crop prices are high or insecticide costs are low, nor is your crop more sensitive to insect damage (remember the damage boundary).

Yield loss occurs at the same level of pest population, regardless of the market prices of commodities. It makes no sense to treat if there’s no reasonable likelihood of damage.

A fixed threshold

It’s best to view the soybean aphid ET as a fixed action or treatment threshold, unlike some more flexible thresholds for other pests.

In the case of soybean aphid, raising the threshold reduces lead time for applications and increases risk of economic loss from rapidly increasing aphid populations. Lowering the threshold may provide a bit more lead time for insecticide application.

However, it also reduces the chances for natural enemies and the soybean field’s sometimes-harsh environment to solve the problem for you. Lowering the threshold reduces your ability to treat only those fields facing a reasonably high risk for yield loss. Ultimately, a sliding scale that lowers the ET isn’t recommended.

Research: Treating below the ET

In university research across the north-central United States, treating below 250 aphids per plant resulted in no observable yield increase, supporting the conclusion that very low thresholds or zero tolerance of aphids isn’t necessary.

Again, no published, peer-reviewed data show that soybean aphid damage is likely below the ET. For a positive return on investment, only treat fields that have a reasonable chance of reaching economically damaging levels.

Added costs

While some may view an insecticide costing “only a couple of dollars” as inexpensive compared to other production inputs, it’s still an added cost for no added benefit. These inputs add up with each acre applied.

Lowering the ET below 250 aphids per plant won’t save yield. Instead, you’ll unnecessarily spend money on many more fields that wouldn’t have had economic loss from aphid injury.

This threshold is conservative in that it gives producers plenty of time for action before yield loss could begin. This is particularly true during late soybean growth stages.

Elimination of natural enemies

While some newer insecticides target a narrower range of insects, most insecticide applications aren’t specific. They’ll kill beneficial insects (lady beetles, parasitic wasps, etc.) as well as pests, later allowing soybean aphid populations to rebound in fields without those beneficial insects to slow them down.

Using the ET gives natural enemies a chance to suppress the aphid population and possibly prevent it from reaching economically damaging levels (Figure 2). After application, insecticide residues will kill insects for a short time, but insecticide activity invariably declines over time (generally, this is considered a good thing).

False sense of security

With most insecticides registered for soybean aphid control (such as pyrethroids), soybean foliage emerging after treatment isn’t protected. Insecticides that are absorbed and translocated within soybean plants typically move upward only a leaf or two and eventually leave unprotected foliage, especially when applied early in the season.

Applying treatments early can result in a false sense of security and a reduced reliance on scouting. If you don’t detect re-infestation before reaching the EIL, it may reduce yield. If you detect re-infestation, you’ll incur additional insecticide application costs.

Less cost-effective

Early treatment can reduce or eliminate the cost efficiencies of a single, well-timed, threshold-based treatment. Unnecessary insecticide applications do nothing positive for a short-term return on investment.

Importantly, long-term returns can reduce if insecticide resistance becomes fixed in the soybean aphid population. This has happened many, many times in the history of pest management, and managing pesticide-resistant pests is seldom cheap and easy—for example, consider the problems with herbicide-resistant weed control.

All soybean fields aren’t equally likely to have a soybean aphid problem. Geographic, landscape, biological and agronomic factors all influence soybean aphid populations.

Research results generated on commercial and University farms across the north-central United States can help identify when and where to target early- and late-season aphid scouting efforts.

Location in the field

• Early aphid infestations are often found in smaller fields near buckthorn, and they’re often more abundant near field edges.

• Soybean aphids prefer moderately dry soil moisture conditions.

• Soybeans grown in soils testing low in potassium contain higher levels of amino acids favorable for soybean aphid development. Aphid feeding can intensify potassium deficiency symptoms on these soils.

• Aphids are often most abundant in late-maturing fields.

Location on the plant

• During vegetative growth stages, soybean aphids are often on the upper, newly expanding leaves.

• During reproductive growth stages, soybean aphids tend to move to leaves, stems and pods lower in the canopy.

• Scout soybean aphids by examining individual plants throughout the field.

• Because soybean aphids remain attached to the plant while feeding and can occur throughout the canopy, using a sweep net isn’t recommended for assessing this pest population.

Population growth

• It takes time for aphid populations to grow.

• Sometimes soybean aphid populations never grow at all—a single aphid doesn’t invariably lead to hundreds! Soybean aphids can initially colonize a field and be rapidly wiped out by a combination of inhospitable environmental conditions and predatory insects.

• Once established, populations can grow at a rate where numbers double every 1.5 days to as many as every six days, depending on environment and natural controls. Research most often documented that in-field populations doubled every three days.

Scouting habits

• Scout soybean fields on a regular basis. Soybean aphid populations can rapidly increase, particularly with winged aphids migrating into fields.

• Focus early-season scouting on fields with high risk for colonization or a history of early colonization by aphids.

• As aphid populations develop, scout more fields. You may not need to visit every field every week, but you may need to scout fields with a history of high populations weekly or more.

How to develop an efficient scouting strategy